Earphones are a type of headphones of smaller size that are placed directly outside of the ear canal without fully enveloping the outer ear. Earphones need to be ergonomically designed in order to universally fit the ears of various users. An earphone encloses a small speaker within it and a wire carries an audio signal to the speaker of the earphone. The casing of the earphone must be designed such that it allows the earphone to be held in the ear canal without the need for any external attachment apparatus. When designing the casing of an earphone there must be a minimum of two parts designed separately that come together to form the earphone. The two parts are a base housing which holds the speaker of the earphone and a rear housing which allows the wire carrying the audio signal to be attached to the earphone and which captures the speaker within the assembled casing.
One problem with the design of an earphone is that if the joining strength of the base housing and the rear housing is not sufficiently strong, the base housing may become detached from the rear housing if the earphone is dropped from a height. An earphone is usually used at eye level of a person which typically would be a height of approximately 1.5-1.8 m. However some earphone casings tend to detach even when dropped from a much lower height of approximately 0.8-0.9 m. Most earphones detach when housings are strained with an impact force of approximately 1.8-2 kg.
As a solution to the above problem some modern earphones use ultrasonic bonding, commonly known as ultrasonic welding to attach the base housing of the earphone to the rear housing. This method permanently seals the earphone casing so that the parts of the housing do not detach when dropped. However the earphone would not be serviceable since the ultrasonic bonding would damage the casing once detached rendering the earphone useless.
Other alternative methods of securing the housing parts, such as the earphone described in U.S. Pat. No. 5,544,253, use a catch locking mechanism where a catch is located on the base housing to lock on to the rear housing of the earphone. The problem with this method is that if the force of the catch locking mechanism is too firm, it will make the two parts difficult to be detached for repairing or servicing purposes. If the force of the catch locking mechanism is too loose the earphone would not be able to withstand the force of a drop from a height. Hence it is difficult to achieve an earphone casing that can withstand a drop test and remain easily removable for repairs by utilizing only a catch locking mechanism.